This invention relates generally to electronic test instruments and in particular to a multimeter having a current sensor for measuring current and test leads for measuring voltage and resistance.
Multimeters, also called digital multimeters or "DMMs", are adapted for measuring a number of parameters generally needed for service, troubleshooting, and maintenance applications. Such parameters typically include a.c. (alternating current) voltage and current, d.c. (direct current) voltage and current, and resistance or continuity. Other parameters such as frequency, capacitance, temperature may be readily added to meet the requirements of the particular application. In order to measure current with a general purpose multimeter, an internal current shunt having a known resistance must be inserted in the current path, requiring a break in the current-carrying conductor. The voltage drop across the current shunt is then measured to determine the current.
General purpose multimeters employing internal current shunts are generally limited to ten amperes maximum because of the capacity of the multimeter test leads and circuitry to carry the current. Furthermore, the multimeter generally must be protected with an internal fuse to prevent excessive current levels from flowing through the multimeter, both for safety reasons and to prevent damage to the multimeter. The difficulty in removing a blown fuse, coupled with the time and cost necessary to procure a replacement fuse, make it desirable to obtain a non-contact current measuring instrument that requires no internal fuse.
Clamp-on multimeters provide improved capability for measuring current over general purpose multimeters by employing an integral current clamp which senses the current in the current-carrying conductor without having to cut or break the current-carrying conductor. The current clamp is typically provided in the same housing with a multimeter which measures other parameters such as voltage and resistance in the conventional manner using separate test probes. The current clamp is closed around the current-carrying conductor, which may include copper wires and buss bars, to sense the magnetic field created by the current flow. The current clamp provides a voltage signal for measurement by the multimeter which calculates and displays the measured current level. Because there is no current shunted from the current-carrying conductor through the clamp-on multimeter, the constraint on the maximum current that may be measured has largely been eliminated. Likewise, the internal fuse has been eliminated in clamp-on multimeters.
In order to obtain a valid current measurement, the magnetic core in the current clamp must completely encircle the current-carrying conductor so that the current clamp is completely closed. The current clamp must be mechanically actuated to open the jaws, the current-carrying conductor inserted, and the jaws then closed around the current-carrying conductor. In tight physical spaces such as an electrical cabinet, inserting the clamp-on multimeter and using this technique to make a current measurement is inconvenient and difficult. The current clamp is subject to breakage, abuse, and is vulnerable to dirt and contamination since the alignment of the jaws allowing completion of the magnetic core is critical to obtaining a valid measurement. Clamp-on multimeters thus suffer from reliability problems as well as requiring a large physical space in which to actuate the jaws of the current clamp. Furthermore, clamp-on multimeters tend to be physically heavy because of the substantial amount of iron used on the magnetic core.
Ammeters having a C-shaped current sensor with a fixed recess for receiving a current-carrying conductor presently exist for measuring electrical current in conductors. The Universal Clamp Meter ("UCM") Model 200 from Multi of Japan has a C-shaped current sensor on one end of its housing and a current clamp on the other end. The UCM is a hybrid instrument illustrating the desirability of having a C-shaped current sensor to overcome the short-comings of the current clamp discussed above. However, the current clamp, despite its disadvantages, was not eliminated from the UCM. The C-shaped current sensor is not compensated, thus making measurement accuracy relatively poor and highly dependent on the geometry of the current-carrying conductor within the fixed recess. The measurement accuracy of the current clamp, which is not substantially affected by the geometry of the current-carrying conductor, far exceeds that of the C-shaped current sensor as implemented in the UCM, thus restricting the C-shaped current sensor to a more limited role of current detection rather than current measurement while the current clamp is then deployed to obtain more accurate current measurements.
In U.S. Pat. No. 5,057,769, "AC Current Sensor", issued Oct. 15, 1991, to Gerald L. Edwards, and assigned to Sensorlink Corporation, a C-shaped current sensor having compensating coils for substantially increased measurement accuracy is described. The Ampstik Current Measuring Products manufactured by SensorLink Corporation, allow for measuring current flowing through high voltage lines without having to physically encircle the current-carrying conductor. The ammeter consists of the C-shaped current sensor with compensating coils coupled to an electronic measurement circuit mounted in an integral housing. In its intended application, the ammeter is placed on the end of a "hot stick", a non-conducting rod used in the electrical power industry for use with high voltage power lines, in order to make a current measurement.
The ammeter must be physically large in order to accommodate the two and four inch buss bars encountered in the electrical utility applications. At the same time, the Ampstik ammeters are optimized to operate in voltage environments of up to 400,000 Volts and can measure currents of up to 5,000 Amperes. Thus, the Ampstik ammeters are not intended for use as handheld multimeters, do not have the sensitivity to operate at the current levels used in typical service and maintenance applications, and have a substantially larger physical size than the prior art clamp-on multimeters discussed above.
In Co-pending application Ser. No. 08/819,527, "Improved Coil for An AC Current Sensor", filed Mar. 17, 1997, to Charles R. Jensen, and assigned to Fluke Corporation, an improved current sensor having reduced size and increased sensitivity and measurement accuracy is described. The current sensor allows for current measurements to be made without having to physically encircle the current-carrying conductor while being adapted for operation in the lower voltage and current ranges that may be encountered in service and maintenance applications for multimeters. The use of compensation coils provides for accurate measurements which are not substantially affected by the geometry of the current carrying conductor within the fixed recess of the current sensor. Magnetic fields outside of the fixed recess of the coil are rejected to a desired degree to prevent interference from other current-carrying conductors.
Therefore, it would be desirable to provide a handheld multimeter having a single current sensor fixed recess to obtain a substantially reduced size and increased reliability.